As application of X-ray CT, there is a technique of decomposing kinds, atomic numbers, densities, and the like of materials included in a subject based on projection data that corresponds to a plurality of energy bins, by using a fact that X-ray absorption properties differ by materials. This is called material decomposition. As the monochromaticity of a divided energy bin increases, the difference in interaction between an identified material and other materials increases. Therefore, to perform the material decomposition with high accuracy, it is preferable that an X-ray having high monochromaticity, that is, an X-ray having a narrow energy bin be used.
On the other hand, if an X-ray having a narrow energy bin or a low dose of X-ray is used to reduce the exposure dose of a subject, the number of photons decreases, and therefore, it becomes more likely to be affected by a noise. Accordingly, in the conventional material decomposition using multi-energy CT, it has been difficult to perform material decomposition with high accuracy when there is much noise due to a low dose.